Dear Michael:
Your observation about the vigor of F1 hybrids and the fall-off of vigor
increase after subsequent crosses with one or the other true-breeding
parent type is a good general observation - depending, of course, upon
what traits help the plant survive in its particular environment.
True breeding (parent) strains of plants are often less vigorous than F1
hybrids because they have been INBRED with members of the same "strain"
to achieve, or even exaggerate a desired trait. The problem with
inbreeding is that the effects of bad genes will sometimes be exaggerated
also. Dog breeders have to deal with this all the time. It's also the
reason you shouldn't marry your sister. Marry someone else if you want
vigorous offspring.
With regard to the second question - remember that HYBRIDS have some
genes of both original parents, though not necessarily in equal numbers.
If the environment becomes more like the one which favored parent strain
A, then hybrids with the more genes of strain A are more likely to
survive and bear offspring. Those genes will be SELECTED to survive by
the environment. Plants which lack the genes necessary to survive (lack
the "selected" A genes) will die or produce few offspring. So, no
mutation is necessary for a regression of the population to a parent
type. The parent genes are already right there in the population of
hybrids.
If you could regain the parent strains through the selection
process, and conditions were the same as in the first hybridization
event, then the answer would be YES - the New hybrids would have the same
increase in vigor as the old plants. (There are many ifs, ands, and
buts. I could explain more if you want.)
I hope this helps and wasn't too confusing! Take care.
Ed Neubauer
neubauer at umd5.umd.edu